There are many known types of Ni based rechargeable alkaline cells such as nickel cadmium ("NiCd"), NiMH, nickel hydrogen, nickel zinc, and nickel iron ("NiFe") cells. At one time NiFe and then NiCd batteries were the most widely used. Just as NiFe batteries were displaced by NiCd batteries, NiCd batteries have now been steadily replaced in all applications by NiMH cells. Compared to NiCd cells, NiMH cells made of synthetically engineered materials have superior electrochemical performance parameters, such as specific energy and energy density, and contain no toxic or carcinogenic elements, such as Cd, Pb, and Hg. For purposes of this patent application, the terms "batteries" and "cells" will be used interchangeably when referring to one cell; although the term "battery" can also refer to a plurality of electrically interconnected cells.
While the present discussion focuses on NiMH batteries, it should be understood that the modified nickel hydroxide materials of the present invention can be used in all types of batteries using nickel hydroxide based positive electrode materials. The term "utilization" will be employed in this disclosure to describe the instant invention in the manner well accepted by those ordinarily skilled in the electrochemical art. As used herein "utilization" will refer to the percentage of the electrons of the nickel hydroxide positive electrode electrochemically transferred during the charge/discharge cycling of the electrode relative to the total number of nickel atoms present in the nickel hydroxide material.
In general, NiMH cells employ a negative electrode made of hydrogen storage alloy that is capable of the reversible electrochemical storage of hydrogen. NiMH cells, also employ a positive electrode made from nickel hydroxide active material. The negative and positive electrodes are spaced apart in the alkaline electrolyte. Upon application of an electrical potential across a NiMH cell, water is dissociated into one hydroxyl ion and one hydrogen ion at the surface of the negative electrode. The hydrogen ion combines with one electron and diffuses into the bulk of the hydrogen storage alloy. This reaction is reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron.
The development of commercially viable NiMH batteries began in the 1980s by improving the negative electrode materials by making them "disordered" as taught by Ovshinsky, et al in U.S. Pat. No. 4,623,597. Such disordered negative electrode materials represented a total departure from other teachings of the period that urged the formation of homogeneous and single phase negative electrodes. (For a more detailed discussion, see U.S. Pat. Nos. 5,096,667; 5,104,617; 5,238,756; 5,277,999; 5,407,761 and 5,536,591 and the discussion contained therein. The disclosure of these patents are specifically incorporated herein by reference.)
The use of disordered negative electrode metal hydride materials significantly increases the reversible hydrogen storage characteristics required for efficient and economical battery applications, and results in the commercial production of batteries having high density energy storage, efficient reversibility, high electrical efficiency, bulk hydrogen storage without structural change or poisoning, long cycle life, and deep discharge capability.
As discussed in U.S. Pat. No. 5,348,822, nickel hydroxide positive electrode material in its most basic form has a maximum theoretical specific capacity of 289 mAh/g, when one charge/discharge cycles from a .beta.II phase to a .beta.III phase and results in one electron transferred per nickel atom. It was recognized in the prior art that greater than one electron transfer could be realized by deviating from the .beta.II and .beta.III limitations and cycling between a highly oxidized .gamma.-phase nickel hydroxide phase and either the .beta.III phase and/or the .varies.-phase. However, it was conventionally recognized dogma that such gamma phase nickel hydroxide formation destroyed reversible structural stability and therefore cycle life was unacceptably degraded. A large number of patents and publications in the technical literature disclosed modifications to nickel hydroxide material designed to inhibit and/or prevent the destructive formation of the transition to the .gamma.-phase.
Attempts to improve nickel hydroxide positive electrode materials began with the addition of elements to compensate for what was perceived as the inherent problems of the material. The use of compositions such as NiCoCd, NiCoZn, NiCoMg, and their analogues are described, for example, in the following patents:
U.S. Pat. No. Re. 34,752, to Oshitani, et al., reissued Oct. 4, 1994, describes a nickel hydroxide active material that contains nickel hydroxide containing 1-10 wt % zinc or 1-3 wt % magnesium to suppress the production of .gamma.-NiOOH. The invention is directed toward increasing utilization and discharge capacity of the positive electrode. Percent utilization and percent discharge capacity are discussed in the presence of various additives.
Oshitani, et al. describe the lengths that routineers in the art thought it was necessary to go to in order to prevent the presence of substantial amounts of .gamma.-NiOOH. The patent states:
Further, since the current density increased in accordance with the reduction of the specific surface area, a large amount of higher oxide .gamma.-NiOOH may be produced, which may cause fatal phenomena such as stepped discharge characteristics and/or swelling. The swelling due to the production of .gamma.-NiOOH in the nickel electrode is caused by the large change of the density from high density .beta.-NiOOH to low density .gamma.-NiOOH. The inventors have already found that the production of .gamma.-NiOOH can effectively be prevented by addition of a small amount of cadmium in a solid solution into the nickel hydroxide. However, it is desired to achieve the substantially same or more excellent effect by utilizing additive other than the cadmium from the viewpoint of the environmental pollution."
U.S. Pat. No. 5,366,831, to Watada, et al., issued Nov. 22, 1994, describes the addition of a single Group II element (such as Zn, Ba, and Cd) in a solid solution with nickel hydroxide active material. The Group II element is described as preventing the formation of gamma phase nickel hydroxide thereby reducing swelling, and the cobalt is described as reducing the oxygen overvoltage thereby increasing high temperature charging efficiency. Both oxygen overvoltage and charge efficiency are described as increasing with increasing cobalt.
U.S. Pat. No. 5,451,475, to Ohta, et al., issued Sep. 19, 1995, describes the positive nickel hydroxide electrode material as fabricated with at least one of the following elements added to the surface of the particles thereof: cobalt, cobalt hydroxide, cobalt oxide, carbon powder, and at least one powdery compound of Ca, Sr, Ba, Cu, Ag, and Y. The cobalt, cobalt compound, and carbon are described as constituents of a conductive network to improve charging efficiency and conductivity. The powdery compound is described as adsorbed to the surface of the nickel hydroxide active material where it increases the overvoltage, for evolution of oxygen, thereby increasing nickel hydroxide utilization at high temperature. Ohta, et al. claims that increased energy storage in NiMH cells using the disclosed invention remains constant up to a high number of charge/discharge cycles and capacity does not drop as much at higher temperatures as it does in cells that do not embody the invention.
U.S. Pat. No. 5,455,125 to Matsumoto, et al., issued Oct. 3, 1995, describes a battery having a positive electrode comprising nickel hydroxide pasted on a nickel foam substrate with solid solution regions of Co and salts of Cd, Zn, Ca, Ag, Mn, Sr, V, Ba, Sb, Y, and rare earth elements. The addition of the solid solution regions is intended to control the oxygen overvoltage during charging. The further external addition of "electric conducting agents" such as powdered cobalt, cobalt oxide, nickel, graphite, "and the like," is also described. Energy density is shown as constant at 72 Wh/kg at 20.degree. C. and 56 Wh/kg at 45.degree. C. for embodiments of the invention over the life of the NiMH cell.
U.S. Pat. No. 5,466,543, to Ikoma, et al., issued Nov. 14, 1995, describes batteries having improved nickel hydroxide utilization over a wide temperature range and increased oxygen overvoltage resulting from the incorporation of at least one compound of yttrium, indium, antimony, barium, or beryllium, and at least one compound of cobalt or calcium into the positive electrode. Cobalt hydroxide, calcium oxide, calcium hydroxide, calcium fluoride, calcium peroxide, and calcium silicate aria specifically described compounds. Additionally described additives are cobalt, powdery carbon, and nickel. The specification particularly describes AA cells using a positive electrode containing 3 wt % zinc oxide and 3 wt % calcium hydroxide as superior in terms of cycle life (250 cycles at 0.degree. C., 370 cycles at 20.degree. C., and 360 cycles at 40.degree. C.) and discharge capacity (950 mAh at 20.degree. C., 850 mAh at 40.degree. C., and 780 mAh at 50.degree. C.).
U.S. Pat. No. 5,489,314, to Bodauchi, et al., issued Feb. 6, 1996, describes mixing the nickel hydroxide positive electrode material with a cobalt powder compound followed by an oxidation step to form a beta cobalt oxyhydroxide on the surface of the nickel hydroxide powder.
U.S. Pat. No. 5,506,070, to Mori, et al., issued Apr. 9, 1996, describes nickel hydroxide positive electrode material containing 2-8 wt % zinc mixed with 5-15 wt % cobalt monoxide. The zinc reduces swelling and the cobalt increases utilization. The capacity of the resulting electrode is stated as being "improved up to 600 mAh/cc" without further description.
U.S. Pat. No. 5,571,636, to Ohta, et al., issued Nov. 5, 1996, describes the addition of at least one powdery compound of Ca, Sr, Ba, Cu, Ag, and Y to the surface of nickel hydroxide active positive electrode material. This patent states that these compounds are adsorbed to the surface of the nickel hydroxide active material creating a conductive network that increases the oxygen overvoltage and improves utilization of the active material at high temperatures. Increased capacity in NiMH cells using the '636 invention remains constant up to a large number of cycles and utilization does not drop as much at higher temperatures as it does in cells that do not embody the invention.
In all of the prior art, the basic nickel hydroxide material is treated, most commonly, by the addition of a single element or a compound thereof, usually Co, to increase conductivity and usually one other element or a compound thereof, usually Cd or Zn, to suppress and/or prevent .gamma.-phase formation. The mechanisms for the asserted improvements in all the above patents are attributable to the following effects:
1. Improved speed of activation, resistance to poisons, and marginal capacity improvement via increased utilization. At the present time, most commercial nickel metal hydride batteries achieve these effects through the addition of up to 5 wt % cobalt. A noted researcher, Delmas, in the Proceeding of the Symposium on Nickel Hydroxide Electrode 118-133 (1991) observed that much higher capacity could result if as much as 20% trivalent cobalt was used. However, even setting environmental and cost considerations aside, the addition of 20% Co is unstable and thus not applicable to commercially viable systems. Frequently, powdered carbon, powdered cobalt metal, and powdered nickel metal are externally also added to create separate conductive networks and thereby improve utilization. Of course, a major drawback of increasing the amount of such elements that are added is that the amount of active nickel hydroxide electrode material is correspondingly reduced, thereby reducing capacity of the electrode. Further, since Co is very expensive, the addition of Co increases cost.
2. Cycle life is extended by decreasing swelling that is initiated by density changes between the oxidized and reduced states of the nickel hydroxide material. Swelling, in turn, is accelerated by the uncontrolled density changes between .beta.II-.beta.III phase nickel hydroxide and .alpha.-.gamma. or .beta.II-65 phase nickel hydroxide. Cd and Zn incorporated into the nickel hydroxide effectively reduce the swelling by reducing the difference in density in the charged and discharged material and increasing the mechanical stability of the nickel hydroxide material itself. This is accomplished by promoting oxygen evolution and thereby reducing charge acceptance which prevents the nickel hydroxide material from attaining the highly oxidized state (the .gamma.-phase state). However, by suppressing or at least significantly inhibiting .gamma.-phase state formation, the nickel hydroxide is limited to low utilization. Further, in order to effectively inhibit .gamma.-phase nickel hydroxide, it is necessary to employ a relatively high wt % of the inhibitor element such as Zn, which high percentage results in a greatly reduced amount of active material being present thereby resulting in reduced electrochemical capacity.
3. The aforementioned "safety release" mechanism of oxygen evolution to avoid highly oxidized states (.gamma.-phase) of nickel hydroxide material actually is an impediment to high temperature operation because a significant increase in the rate of oxygen evolution occurs with increasing temperature. The effect of such increased oxygen evolution is a very substantial decrease in utilization and ultimately a reduction in energy storage at higher temperatures in the NiMH battery using these materials. At 55.degree. C., for example, run times of a battery may be reduced by 35-55% compared to the room temperature performance of that same battery.
Elevated operational temperature conditions aside, none of these modifications of the active positive electrode material suggested by the prior art result in more than an incremental improvement in performance and none result in a significant increase in the capacity and/or utilization of the nickel hydroxide material itself, even at room temperature. All prior art batteries are limited to less than one electron transfer per nickel atom. Further, these modifications fail to address the special operational requirements of NiMH batteries, particularly when NiMH batteries are used in electric vehicles, hybrid vehicles, scooters and other high capacity, high drain rate applications. Because NiMH negative electrodes have been improved and now exhibit an extremely high storage capacity, the nickel hydroxide positive electrode material is essentially the limiting factor in overall battery capacity. This makes improving the overall electrochemical performance of the nickel hydroxide material in all areas more important than in the past. Unfortunately, the elements currently added or previously suggested to be added to the nickel hydroxide material result in insufficient improvements in performance before competing deleterious mechanisms and effects occur. For example, Cd cannot be used in any commercial battery because of the environmental impact thereof, and Co and Zn appear to become most effective only at levels that result in a significant decrease in cell capacity; more specifically, energy per electrode weight.
Accordingly, there remains a long felt need in the art for an improved, higher capacity, higher utilization, high temperature nickel hydroxide positive electrode for a nickel metal hydride battery.